1. Field of the Invention
The present invention relates to a liquid crystal display (LCD) device and more particularly, to a LCD device with a wide viewing angle.
2. Discussion of the Related Art
In general, a liquid crystal display (LCD) device includes two substrates that are spaced apart and face each other with a liquid crystal layer interposed between the two substrates. Each of the substrates includes electrodes that face each other, wherein a voltage applied to each electrode induces an electric field between the electrodes and within the liquid crystal layer.
The LCD device includes various liquid crystal modes. The liquid crystal mode used also drives whether an optical film is needed as well as what type of optical film is needed.
The liquid crystal layer includes a dielectric anisotropic material. Accordingly, when an electric field is applied to the liquid crystal layer, the liquid crystal molecules form a dipole due to the spontaneous polarization. Thus, the liquid crystal molecules of the liquid crystal layer are arranged by the applied electric field. Optical modulation of the liquid crystal layer occurs according to the arrangement of the liquid crystal molecules. Therefore, images are produced and displayed by the LCD device by controlling light transmittance of the liquid crystal layer due to optical modulation.
FIG. 1 is an exploded perspective view of a liquid crystal display (LCD) device according to the related art. In FIG. 1, an LCD device 51 has upper and lower substrates 5 and 22, which are spaced apart from and facing each other, and a liquid crystal layer 11 interposed between the upper and lower substrates 5 and 22.
The upper substrate 5 includes a black matrix 6, a color filter layer 7, and a transparent common electrode 9 subsequently disposed on an interior surface thereof. The black matrix 6 has an opening such that the color filter layer 7 corresponds to the opening of the black matrix 6 and includes three sub-color filters of red (R), green (G), and blue (B).
Gate lines 12 and data lines 24 are formed on an interior surface of the lower substrate 22, whereby gate lines 12 and date lines 24 cross each other to define pixel areas P. A thin film transistor T is formed at the crossing of a gate line 12 and a data line 24. The thin film transistor T is composed of a gate electrode, a source electrode, and a drain electrode. The thin film transistors of respective gate line 12 and data line 24 crossings are arranged in a matrix. A pixel electrode 17, which is connected to the thin film transistor T, is formed within a pixel area P and corresponds to the sub-color filters. In addition, the pixel electrode 17 is made of a transparent conductive material, such as indium-tin-oxide (ITO). The lower substrate 22 may be commonly referred to as an array substrate.
In operation, a scanning pulse is supplied to the gate electrode of the thin film transistor T through the gate line 12, and a data signal is supplied to the source electrode of the thin film transistor T through the data line 24.
However, the above-mentioned LCD device has a disadvantage of a narrow viewing angle. To overcome the narrow viewing angle, various methods, such as a multi-domain method, a phase compensation method, an in-plane switching (IPS) mode, and a vertical alignment (VA) mode, have been researched and developed.
In the multi-domain method, a pixel is divided into several regions, in each of which liquid crystal molecules are differently arranged, and the pixel has average properties of the regions. In the phase compensation method, a phase difference film, which may be referred to as a retardation film, is used to reduce changes in phase difference depending on viewing angles. In the IPS mode, liquid crystal molecules move in a plane substantially parallel to the substrates according to an electric field parallel to the substrate of the LCD device. In the VA mode, liquid crystal molecules having negative dielectric anisotropy are arranged vertically with respect to the substrate by a vertical alignment layer when voltage is not applied.
Among these methods, the VA mode has an additional advantage of fast response time as compared with twisted nematic (TN) mode, which is widely used in conventional LCD devices, because of small changes of response time to gray scale. The VA mode has a response time of about 30 ms as compared with the 50 ms of the TN mode, when the transmittance of the LCD device changes from 100% to 50%.
Generally, in the VA mode, a vertical alignment material, a liquid crystal material with negative dielectric anisotropy and a negative retardation film are used. Thus, the VA mode has a wide viewing angle, and has a high contrast ratio.
FIG. 2 is a cross-section view of a VA mode LCD device according to the related art; and FIG. 3 shows a state of the VA mode LCD device to be displayed.
As shown in the figures, a pixel electrode 17 is formed on an interior surface of a first substrate 22, and a black matrix 6, a color filter layer 7 and a common electrode 9 are subsequently formed on an interior surface of a second substrate 5, which is spaced apart from and facing the first substrate 22. A liquid crystal layer 11 is interposed between the first and second substrates 22 and 5. The liquid crystal layer 11 has a negative dielectric anisotropy, and liquid crystal molecules of the liquid crystal layer 11 may be vertically arranged between the first and second substrates 22 and 5. The pixel electrode 17 is patterned according to correspond to the pixel areas.
When voltage is applied to the pixel electrode 17 and the common electrode 9, electric field E substantially perpendicular to the substrates 5 and 22 is induced between the pixel electrode 17 and the common electrode 9, and the liquid crystal molecules of the liquid crystal layer 11 are arranged substantially perpendicular to the electric field E. At this time, in edges of the pixel electrode 17, the electric field may be bent a bit.
By the way, as shown in FIG. 3, the liquid crystal molecules of the VA mode appear irregular in each pixel P1, P2, and P3 if alignment layers (not shown) are not treated by an alignment treatment such as rubbing, or slit or hole is not formed in the pixel, for example. Thus, a viewing angle is unstable, and the response time is reduced. To widen and stabilize the viewing angle, the multi-domain method may be included within the VA mode by inducing fringe field around a slit or a hole of the pixel electrode.
An embodiment of the multi-domain by the hole is disclosed in U.S. Pat. No. 6,100,953, which is hereby incorporated by reference for all purposes as if fully set forth herein and described with respect to FIGS. 4A and 4B and FIG. 5. FIG. 4A is a schematic cross-sectional view of another VA mode LCD device according to the related art; and FIG. 4B is a plan view of a color filter substrate corresponding to the VA mode LCD device of FIG. 4A. FIG. 5 shows a pixel including multiple domains according to the related art.
As shown in the figures, a pixel electrode 17 is formed on an interior surface of a first substrate 22. A second substrate 5 is spaced apart from the first substrate 22, and a black matrix 6, a color filter layer 7 and a common electrode 9 are subsequently formed on an interior surface of the second substrate 5. The color filter layer 7 has a hole 30 exposing the second substrate 5 therein.
When voltage is applied to the pixel electrode 17 and the common electrode 9, electric field E is induced between the pixel electrode 17 and the common electrode 9. Here, the induced electric field E has different distribution around the hole 30. That is, since the electric field E is distorted by the hole 30, fringe field is formed around the hole 30. Therefore, multi-domains are formed in a pixel, as shown in FIG. 5.
In FIG. 5, the liquid crystal molecules are arranged uniformly in each domain A, B and C, and the LCD device are stably displayed with symmetry. However, the VA mode LCD device has still a problem of low brightness. This problem will be explained hereinafter with reference to FIG. 6. FIG. 6 schematically shows a plan view of the VA mode LCD device according to the related art when voltage is applied to the liquid crystal material.
In FIG. 6, liquid crystal molecules 11 are disposed between first and second polarizers, wherein optical axes 50 and 52 of the polarizers cross at right angles. The second polarizer, from which light is emitted, is usually referred to as an analyzer. The liquid crystal molecules 11 have negative dielectric anisotropy. As illustrated in FIG. 6, when voltage is applied to electrodes (not shown), the liquid crystal molecules 11 are arranged such that long axes of the liquid crystal molecules 11 are horizontal, that is, parallel to the substrates. By the way, the liquid crystal molecules 11 in regions F and G are parallel to the optical axes 50 and 52 of the first and second polarizers, respectively. Thus, light is not emitted in the regions F and G, and black state regions are formed in a white image. Since the black regions are shown on the substrate of the LCD device, white brightness is lowered.
To solve the problem, a VAC (vertical alignment with chiral dopants) mode, wherein chiral dopants are mixed with liquid crystal molecules to be vertical aligned, has been proposed, as shown in FIG. 7. FIG. 7 schematically shows a plan view of a VAC mode LCD device according to the related art when voltage is applied to liquid crystal molecules.
As shown in FIG. 7, the liquid crystal molecules 11 have a twisted structure between first and second polarizers due to a helical characteristic of chiral dopants. Thus, in regions F and G, where the liquid crystal molecules 11 are parallel to optical axes 50 and 52 of the polarizers, respectively, light is emitted due to birefringence of the liquid crystal molecules 11, and brightness is improved.
FIG. 8 shows transmittance (that is, brightness) versus voltage characteristics of a related art VA mode and a related art VAC mode. As shown in FIG. 8, the VAC mode has higher transmittance than the conventional VA mode at the same voltage.